Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross section. The two main advantages of this process over other manufacturing processes are its ability to create very complex cross sections and work materials that are brittle, because the material only encounters compressive and shear stresses.
Hollow cavities within extruded material cannot be produced using a simple flat extrusion die, as there would be no way of supporting the center barrier of the die. Alternatively, the die assumes the shape of a block with depth, beginning first with a shape profile that supports the center section. The die shape then internally changes along its length into the final shape, with the suspended center pieces supported from the back of the die.
Once the polymeric tube egresses from the die, the polymeric tube, being no longer supported by the die, may have a tendency to deform or collapse.